All optical dew point hygrometers detect the onset of condensation and measure the temperature of the surface where the condensation occurs. All optical dew point hygrometers are susceptible to contaminants collecting on mirror surfaces, which cause a reduction in density of the operating dew layer. This generally reduces the accuracy of the dew point measurement to a degree which depends on the amount of contaminant present and its solubility in water. Both soluble and insoluble materials, if allowed to build up on the condensing surface, will eventually cause the system to go out of control because of reduced mirror reflectance. It has been a common practice to address the latter (control) problem by either manually or automatically disabling the feedback control system which controls the mirror (condensing surface) temperature and forcing the mirror surface to heat to a dry state, at which time, an additional current is injected into the control loop amplifier.
Unfortunately contaminants encountered by optical dew point hygrometers vary widely. One of the most common contaminants are soluble salts in the atmosphere. These salts range from sea salts in coastal areas to limestone in similar geological formations inland. They are always present in the atmosphere and gradually build up on the mirror surface. In addition, these natural contaminants are accompanied and often exceeded by other man-made soluble contaminants, including products of the process under measurement. In the prior art systems, heating of the mirror to the dry state for manual or automatic rebalancing of the optical detection circuit overcomes the loop offset problem associated with reduced reflectance but does not address the problem of measurement error associated with soluble matter which remains. The soluble materials such as salts precipitate out and form a thin layer on the mirror surface. The salts tend to absorb water vapor at temperatures above the dew point and dissolve back into the dew layer when the mirror recools. However, the contaminated mirror does not reach the true dew point even after compensating for the reduced reflectance. The resultant dew layer contains salts which cause the saturation vapor pressure of the dew deposit to decrease, this is in accordance with Raoult's Law. In the absence of soluble contaminants, saturation vapor pressure is a function of temperature only. The dew point is by definition, that temperature where the saturation vapor pressure of the liquid is equal to the partial pressure of the water vapor in the air or carrier gas which is in contact with the liquid. The partial pressure is dependent upon vapor concentration and total pressure only. The equilibrium temperature (dew point) is therefore a true measure of water vapor concentration in the absence of soluble materials, when the total pressure is known. The presence of solute at the site of the condensate causes the instrument to control at a temperature higher than the true dew pont. The instrument then displays a higher than actual dew point temperature reading. In a mildly contaminated environment such as might be encountered in an air conditioned area pulling lithium chloride dryers, a typical optical dew point hygrometer will gradually drift upwards, exhibiting an error approaching as much as 3.degree.-4.degree. C. after a few weeks of continuous operation without mirror cleaning. Therefore, the mirror must be periodically cleaned, the interval being selected as a function of the contamination rate and accuracy desired. Usually, cleaning the mirror once a week is adequate to keep the error band of the instrument. In certain instances, where the contamination levels are high, daily cleaning is required. It is therefore a primary object of this invention to provide an instrument where the time required between mirror cleaning for a given rate of contamination and measurement error is extended by orders of magnitude over the required interval for mirror cleaning for prior art instruments.
It is a further object of this invention to provide an instrument where the initial accuracy of the dew-point temperature is maintained despite contamination of the mirror by water soluble materials between cleanings of the mirror.
A further object of this invention is to provide accurate control of the density of the operating dew layer over the specified range of operating conditions.
It is a still further object of the invention to provide automatic compensation of the temperature control loop to eliminate the effects of mirror contamination or variations in the light emitting diodes or photodetectors on the operation of the temperature control loop.
A still further object is to provide totally automatic and unattended operation. All of the operator adjustable parameters; loop gain, dew thickness, loop frequency compensation, and heat pump current limiting which existed in the prior art apparatus are automatically controlled. The manual controls have therefore been eliminated.
It is a feature of this invention that the dew point measurement operation is periodically interrupted by disabling the temperature control servomechanism with the measured dew point being stored in a memory circuit so as not to interrupt the dew point data display.
It is a further feature of this invention that a signal representative of the reduction in mirror reflectance is stored to provide reflectance compensation for said reduction in reflectance during dew point measurement operation. This feature is provided in order that the apparatus be capable of establishing the required operating density of condensate when the instrument is in control at the dew point temperature.